Modularity in MVVM light toolkit ?

MVVM Light is a toolkit providing components that help to build loosely coupled. It also pays a lot of attention to facilitating the designer-developer workflow by optimizing the interaction with Expression Blend.

I would not compare MVVM Light to Prism. First, Prism is much more complex and has more features than MVVM Light. Their aim is also not quite the same.

Some features of Prism can be replaced by a combination of the Managed Extensibility Framework MEF and MVVM Light. Similarly, some features of MVVM Light can be fulfilled by using components of Prism.